Yes, Aesop Story Engine looks really nice on their premium themes, but all the base functionality is free.
Just since seeing this post I spun up a new WordPress 4.0 site using the default Twenty Fourteen theme and installed Aesop Story Engine. Works great without my paying for the plugin, without my paying someone to develop the plugin for me, and without my spending the probably dozens of hours it would take me to make the plugin myself.
In fact I’m planning on installing it on a site for my wife later this week. I’ll probably build my own theme for it, but in no way does Nick owe me a free premium theme after he and other contributors chose to basically donate a huge chunk of time to give me (and you) something for free.
If you donate time or money to a charity you would be pretty offended if they then expected you to give the rest of your time and money to them because you showed interest. It’d be your choice whether to support that charity further, and it’s your choice whether to support Aesop Story Engine further by purchasing premium themes and addons.